A baseline definition of the mechanical properties and related chemical composition of the vascular smooth muscle of the lung is sought in order to gain an understanding of the mechanisms of regulation of the pulmonary vasculature in normal and disease states, and an understanding of its responsiveness to changes in the internal environment. The major objectives are to determine the active and passive mechanical characteristics of intrapulmonary muscular arteries and veins of the fetus and adult, through the determination of their length-tension and force-velocity relationships, and an analysis of the viscoelastic behavior originating in connective tissue and contractile elements. We will further characterize the vessels on the basis of their cell size and number, contractile protein and scleroprotein content, phosphagen content, relative representative oxidative and glycolytic enzyme activities and finally the chemical energetics of force maintenance, in order to explain the basis of changes in functional behavior.